This type of poppet valves can be forged from an austenitic heat-resisting steel including Nickel. Such forged valve has a grain size of ASTM (American Society for Testing and Materials) 11 or greater and has an excellent tensile strength and other low-temperature properties. However, in view of the fact that the head portion is exposed to a hot exhaust air, it has been proposed in Patent Document 1 to subject the head portion to a solution heat treatment to thereby coarsen the grains in that portion, thereby securing high-temperature properties (particularly heat resisting property of the face of the head portion).
The Patent Document 1 discloses a one-piece exhaust poppet valve comprising a head portion which has a seat portion formed on the outer periphery thereof and is integral with a diametrically tapered neck portion of a linear stem portion, wherein the valve is externally subjected to a solution heat treatment from the face of the head portion so that the head portion has a coarse grain size and desired high-temperature properties, and wherein the valve has finer grains towards the stem portion so that the stem portion has a good low-temperature ductility and a wear resistance. The Patent Document 1 also discloses a method of solution heat treating such valve.    Patent Document 1: Japanese Patent Publication No. 04-027283.